This invention relates to the recovery of hydrocarbons and to the recovery of energy from carbon or hydrocarbon-bearing rocks.
Coal and lignite are normally mined by excavation and oil is produced by drilling oil-bearing rocks. With the depletion of worldwide reserves of liquid-fuel hydrocarbon, there has been much effort to extract hydrocarbon from oil-shales, coals, tar-sands and other carbon and hydrocarbon-bearing rocks. Those rocks can be excavated and subsequently retorted, distilled, or hydrogenated. Processes are known for chemical processing of oil-shales, coals, tar-sands, etc., in factories. The intensive costs of mining and processing make such processes uneconomical as long as liquid-fuel can be obtained cheaply. Furthermore, the environmental problems caused by the mining of large volumes of oil-shale and tar-sands make mining unacceptable.
Current in-situ methods have the advantage of protecting the environment. Technology for in-situ recovery of hydrocarbons from oil-shale, tar-sands, and coal, and for secondary recovery of hydrocarbons from oil-bearing beds have been developed during the last several decades. Hundreds of patents have been issued using processes such as:
(1) Processes to enhance the porosity and permeability of hydrocarbon and carbon-bearing formations so that hydrocarbons could flow or be pumped out from underground. The methods include (a) hydrofracturing, (b) blasting, and (c) undercutting over a large area to cause the collapse of the overlaying deposit into the excavation, or a combination of those; PA1 (2) Processes to inject fluid into injection wells, and thus to provide a hydrodynamic potential to force the injected fluid to displace the hydrocarbons in oil-bearing beds so that the latter can flow into production-wells and then be removed. A most common method of this type of process is secondary recovery by water-flooding; PA1 (3) Processes to provide a heat source such as steam-flooding, or by other means to increase the underground temperature and thus to lower the viscosity of hydrocarbons in oil-bearing beds, tar-sand, or coal sufficiently to flow or be pumped out from underground. The methods are commonly called thermal-stimulations; and PA1 (4) Processes to inject fluid into injection wells, to provide a hydrodynamic potential to force the injected fluid into contact with the carbon or hydrocarbon-bearing rock, producing hydrocarbons which can flow into production wells and be removed.
Current in-situ methods use one or a combination of these processes. Methods for recovering carbonaceous materials from oil-shales, collectively known as "shale-burning" are described in U.S. Pat. Nos. 3,661,423, 4,106,814, 4,109,719, 4,147,389, 4,151,877, 4,158,467 and DE 4,153,110. These are methods of in-situ retorting using a combination of processes (1) and (2). None of the methods are economical at the present, and are not in commercial use.
Other in-situ methods such as steam-flooding, thermal-stimulation, gasification of coal, hydrogenation of tar-sand, in-situ combustion, etc. represent other combinations of those processes (e.g., U.S. Pat. Nos. 4,085,803, 4,089,373, 4,089,374, 4,093,027, 4,088,188, 4,099,568, 4,099,783, 4,114,688, 4,133,384, 4,148,359, 4,149,595, 4,476,932, 4,574,884, 4,598,770, 4,896,345, 5,207,271, 5,360,068 and Int. Publ. No. WO 95/06093). All of those methods require the injection of fluid or insertion of a heat source, via injection wells, directly into the carbon or hydrocarbon-bearing formations and they prescribe the production of hydrocarbons (or hot gases) from production wells. Commonly the wells are vertically drilled into a hydrocarbon-bearing formation, and fluid or heat flows horizontally from well to well. The movement from a point source in the injection well laterally to a production well describes a linear path and such injection methods have a low efficiency when a large part of the host-rock is by-passed.
Methods to increase the efficiency of in-situ methods by drilling wells horizontally or in a direction parallel to a hydrocarbon-bearing formation such as tar-sand or coal, are suggested by U.S. Pat. Nos. 4,410,216, 4,116,275, 4,598,770, 4,610,303, and 5,626,191. Such orientation provides a line source for fluid or heat energy which can penetrate into the surface(s) around the borehole. The shortcoming of the methods is the limited penetration into the hydrocarbon-bearing formation, so that a plurality of holes have to be drilled. Also there is no systematic control of the fluid or heat-flow, its rate, its penetration, etc., or of the condition of in-situ physical conditions, such as temperature, and rate of chemical reaction.
U.S. Pat. No. 4,550,779 suggested that fluid can be induced to flow from one porous and permeable formation vertically into another porous and permeable formation. However, the method cannot be used unless at least a pair of such formations are present. Also the efficacy of the process is limited by the relatively low permeability of natural formations.
An "in-situ chemical-reactor for recovery of metals or purification of salts" is disclosed in our co-pending patent appln. Ser. No. 08/852,327 filed May 7, 1997.
It is an object of the present invention to improve the previously described in-situ reactor and to facilitate physical and chemical changes in coal (including lignites), oil-shale, tar-sand, and other carbonaceous deposits to produce hydrocarbons after the hydrocarbons in those deposits have been made less viscous, or to produce thermal energy in the form of hot combustion products, which can be recovered and converted into other forms of energy, such as electricity.